When an oil well is first drilled and completed, the fluids (such as crude oil) may be under natural pressure that is sufficient to produce on its own. In other words, the oil rises to the surface without any assistance.
In many oil wells, and particularly those in fields that are established and aging, natural pressure has declined to the point where the oil must be artificially lifted to the surface. Subsurface pumps are located down in the well below the level of the oil. A string of sucker rods extends from the pump up to the surface to a pump jack device, or beam pump unit. A prime mover, such as a gasoline or diesel engine, or an electric motor, or a gas engine on the surface causes the pump jack to rock back and forth, thereby moving the string of sucker rods up and down inside of the well tubing.
The string of sucker rods operates the subsurface pump. A typical pump has a plunger that is reciprocated inside of a barrel by the sucker rods. The barrel has a standing one-way valve, while the plunger has a traveling one-way valve, or in some pumps the plunger has a standing one-way valve, while the barrel has a traveling one-way valve. Reciprocation charges a chamber between the valves with fluid and then lifts the fluid up the tubing toward the surface.
One problem encountered in downhole pumps is that the chamber between the valves fails to fill completely with liquid. Instead, the chamber contains undissolved gas, air, or vacuum, which are collectively referred to herein as "gas".
Such failure to completely fill the chamber is attributed to various causes. In a gas lock situation or a gas interference situation, the formation produces gas in addition to liquid. The gas is at the top of the chamber, while the liquid is at the bottom, creating a liquid-to-gas interface. If this interface is relatively high in the chamber, gas interference results. In gas interference, the plunger (on the downstroke) descends in the chamber and hits the liquid-to-gas interface. The change in resistances causes a mechanical shock or jarring. Such a shock damages the pump, the sucker rods and the tubing.
If the liquid-to-gas interface is relatively low in the chamber, gas lock results, wherein insufficient pressure is built up inside of the chamber on the downstroke to open the plunger valve. The plunger is thus not charged with fluid and the pump is unable to lift anything. A gas locked pump, and its associated sucker rods and tubing, may experience damage from the plunger hitting the interface.
In a pump off situation, the annulus surrounding the tubing down at the pump has a low fluid level, and consequently a low fluid head is exerted on the barrel valve. In an ideal pumping situation, when the plunger is on the upstroke, the annulus head pressure forces annulus fluid into the chamber. However, with a pump off condition, the low head pressure is unable to force enough fluid to completely fill the chamber. Consequently, the chamber has gas or air (a vacuum) therein. A pump (and its associated equipment) that is in a pump off condition suffers mechanical shock and jarring as the plunger passes through the liquid-to gas interface. A restricted intake can also cause pump off.
Still another problem is sand. The plunger and the barrel are both made of metal. In order to provide for lubrication inbetween these two parts, a small clearance between the two is provided to allow oil to enter. When the well is producing sand, some sand may enter this clearance or spacing between the plunger and the barrel. The sand abrades the components, thereby shortening the life of the pump.